Bearing for axial stiffening

ABSTRACT

An example of a hub for a tail rotor includes a body configured to couple to a mast of a rotor system, a trunnion disposed within the body, first and second shafts disposed on opposite sides of the trunnion, first and second end plates secured to the body, and first and second end bearings, the first end bearing disposed between the first shaft and the first end plate and the second end bearing disposed between the second shaft and the second end plate.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.16/272,091, filed Feb. 11, 2019, the contents of which are incorporatedby reference in their entirety herein for all purposes.

BACKGROUND

This section provides background information to facilitate a betterunderstanding of the various aspects of the disclosure. It should beunderstood that the statements in this section of this document are tobe read in this light, and not as admissions of prior art.

Helicopters typically include a main rotor that rotates in a generallyhorizontal plane above the helicopter airframe and a tail rotor thatrotates in a generally vertical plane oriented to produce a sidewaysthrust in the direction of yaw. The pitch of the tail rotor blades,i.e., the angle between the chord line of the blade profile and thedirection of rotation of the tail rotor, can be varied so as to increaseor decrease the amount of sideways thrust produced by the tail rotor.The sideways thrust of the tail rotor serves three related purposes:first, since the tail rotor is located on a tail boom a distance fromthe main rotor, its sideways thrust produces a moment which serves tooffset the torque produced on the airframe of the helicopter by therotation of the main rotor blade; second, the sideways thrust of thetail rotor provides yaw axis control for the helicopter; and third, thesideways thrust of the tail rotor may work in conjunction with sidewaysthrust of the main rotor when the helicopter is translating laterallythrough the air.

The total sideways thrust produced by the tail rotor is known as thetail rotor authority. Factors affecting the total authority produced bya tail rotor include blade size and profile, rotational speed, angle ofattack of the tail rotor blades, the pitch of the tail rotor blades, andthe air density. The angle of attack is the angle between the chord lineof the blade profile and the “relative wind”, i.e., the direction atwhich the air approaches the tail rotor blade. This angle of attack isaffected by the rotor blade pitch, the direction of travel of thehelicopter and the presence of cross winds. A cross wind which reducesthe angle of attack reduces the overall authority produced by the tailrotor, diminishing the control available to the pilot. The pitch is theangle between the chord line of the blade profile and the direction ofblade rotation. The pitch is not affected by cross winds. The pilotcontrols the pitch of the tail rotor blades through the use of controlpedals. Increasing the blade pitch results in greater tail rotorauthority and decreasing the blade pitch results in less tail rotorauthority. Air density also affects the tail rotor authority. Otherfactors being equal, the greater the air density, the greater theauthority produced by the tail rotor, and similarly, the lower the airdensity, the less authority produced by the tail rotor.

During operation of a helicopter, various vibrations are generated. Themain rotor and tail rotor systems of a helicopter are designed to avoiddynamic loading issues that can be caused by vibrations (e.g.,resonance) and negatively impact performance of the helicopter. Forexample, dynamic loading issues in a tail rotor system can be avoided bytuning the natural frequency of the tail rotor system. The naturalfrequency of the tail rotor system can be tuned by, for example,altering the design of components within the tail rotor system (e.g.,changing shape, size, or mass of components). Determining the naturalfrequency of a tail rotor system involves complex mathematics thatnecessarily involves assumptions (e.g., at boundary conditions). As aresult, it can be very difficult to precisely design a tail rotor systemthat avoids all dynamic loading issues, such as resonance.

SUMMARY

An example of a hub for a tail rotor includes a body configured tocouple to a mast of a rotor system, a trunnion disposed within the body,first and second shafts disposed on opposite sides of the trunnion,first and second end plates secured to the body, and first and secondend bearings, the first end bearing disposed between the first shaft andthe first end plate and the second end bearing disposed between thesecond shaft and the second end plate.

An example of a hub for a tail rotor includes a body configured tocouple to a mast, a trunnion disposed within the body, first and secondshafts disposed on opposite sides of the trunnion, first and secondelastomeric bearings, the first elastomeric bearing being disposed onthe first shaft and the second elastomeric bearing being disposed on thesecond shaft, first and second mounting rings, the first mounting ringdisposed between the first elastomeric bearing and an inner wall of thebody and the second mounting ring disposed between the secondelastomeric bearing and the inner wall of the body, first and second endplates secured to the body, the first and second end plates comprising adome shape, and first and second end bearings, the first end bearingdisposed between the first shaft and the first end plate and the secondend bearing disposed between the second shaft and the second end plate.

An example of a system for mounting a teetering helicopter rotor onto amast includes a body with an aperture therethrough for receiving themast, a pair of opposed conical shafts extending from the body and on acommon axis which perpendicularly intersects an axis of the aperture,elastomeric bearings comprising conical shims therein, the conical shimsbeing circumferentially disposed around the conical shafts and havingangles that match the angles of the conical shafts, outer mounting ringshaving interior and exterior surfaces, wherein the interior surfaceshave the same cone angle as the angle of the conical shafts and theexterior surfaces contact an inner wall of the body, and end bearingscomprising alternating layers of rubber and metal shims, wherein an axisof the end bearings aligns with the common axis.

An example of a method of improving a rotor system includes providing abody configured to couple to a mast of a rotor system, placing atrunnion within the body, the trunnion comprising first and secondshafts, placing first and second elastomeric bearings on the first andsecond shafts, respectively, placing first and second end bearingsadjacent to the first and second shafts, and securing first and secondend plates to the body so that the first and second end bearings aredisposed between the first and second end plates the first and secondshafts, respectively. The first and second end bearings adjust a springrate of the rotor system along a central axis passing through centers ofthe first and second elastomeric bearings to move the natural frequencyof the rotor system along the central axis away from a fundamentalnatural frequency of the rotor system.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1A and 1B illustrate top and side views, respectively, of ahelicopter according to aspects of the disclosure;

FIG. 2 is a close-up view of a tail rotor system of the helicopter ofFIGS. 1A and 1B according to aspects of the disclosure;

FIG. 3 is a close-up view of a single hub of the tail rotor systemaccording to aspects of the disclosure;

FIG. 4 is a sectioned view of a prior art hub of a hub assembly; and

FIG. 5 is a sectioned view of a hub of a hub assembly according toaspects of the disclosure.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent aspects, or examples, for implementing different features ofvarious embodiments. Specific examples of components and arrangementsare described below to simplify the disclosure. These are, of course,merely examples and are not intended to be limiting. In addition, thedisclosure may repeat reference numerals and/or letters in the variousexamples. This repetition is for the purpose of simplicity and clarityand does not in itself dictate a relationship between the variousembodiments and/or configurations discussed.

In the specification, reference may be made to the spatial relationshipsbetween various components and to the spatial orientation of variousaspects of components as the devices are depicted in the attacheddrawings. However, as will be recognized by those skilled in the artafter a complete reading of the present disclosure, the devices,members, apparatuses, etc. described herein may be positioned in anydesired orientation. Thus, the use of terms such as “above,” “below,”“upper,” “lower,” or other like terms to describe a spatial relationshipbetween various components or to describe the spatial orientation ofaspects of such components should be understood to describe a relativerelationship between the components or a spatial orientation of aspectsof such components, respectively, as the device described herein may beoriented in any desired direction.

FIGS. 1A and 1B illustrate a top view and a side view, respectively, ofa helicopter 100 according to aspects of the disclosure. Helicopter 100includes an airframe 102 with a tail boom 104. Helicopter 100 alsoincludes a main rotor system 106 that includes a plurality of mainrotors 108 and a tail rotor system 110 that includes a plurality of tailrotors 112. Main rotor system 106 and tail rotor system 110 are poweredby a power system 114 that is housed in airframe 102. Power system 114includes at least one engine that provides torque to rotor systems 106,110.

FIG. 2 is a close-up view of tail rotor system 110 according to aspectsof the disclosure. Tail rotor system 110 includes hubs 120, 121, each ofwhich includes a pair of tail rotors 112. A mast 111 extends througheach hub 120, 121 and provides torque from power system 114 thereto.Each tail rotor 112 includes a grip 119 with a pitch horn 122. Each grip119 is coupled to its respective hub such that axial rotation of grip119 (and tail rotor 112) is permitted. Axial rotation of grip 119 iscontrolled by a collective control system 124. Each pitch horn 122 iscoupled to a yoke 126 of collective control system 124 via a linkage128. Axial movement of yoke 126 causes linkages 128 to alter a pitch oftail rotors 112. Altering the pitch of tail rotors 112 varies an amountof thrust generated by tail rotor system 110 and allows a pilot to yawhelicopter 100.

FIG. 3 is a close-up view of hub 120 according to aspects of thedisclosure. Hub 121 is substantially similar to hub 120. Hub 120 will bediscussed with the understanding that the discussion of hub 120 alsoapplies to hub 121. In FIG. 3, some components have been hidden fromview for illustrative purposes. Hub 120 includes a body 123 with arms130, 131 that extend away therefrom. Arms 130, 131 provide mountingpoints to which grips 119 attach. Hub 120 also includes an aperture 132through which mast 111 passes. Hub 120 houses several components,including elastomeric bearings, that are used to support the loadsgenerated by tail rotors 112 during operation of tail rotor system 110.These components will be discussed in more detail relative to FIGS. 4and 5.

FIG. 4 is a sectioned view of hub 120 with a bearing configuration fromthe prior art. A trunnion 134 is positioned within hub 120 and includesa splined opening 136 and shafts 138, 140. Splined opening 136 includessplines 137 that mate with corresponding splines of mast 111 for torquetransmission. Shafts 138, 140 are located on opposite sides of splinedopening 136 and are frustoconical in shape. In other aspects, shafts138, 140 may have other shapes, such as cylindrical with no taper.Elastomeric bearings 142, 144 are seated about shafts 138, 140,respectively. Elastomeric bearing 142 will be discussed with theunderstanding that the discussion of elastomeric bearing 142 alsoapplies to elastomeric bearing 144. Elastomeric bearing 142 comprisesalternating conical layers of rubber and conical metal shims. Thealternating layers form concentric rings with the frustoconical surfacesof shaft 138. In some aspects, elastomeric bearing 142 is adhered to theouter surface of shaft 138.

Mounting rings 146, 148 are adhered to an outer surface of theelastomeric bearings 142, 144, respectively. Mounting ring 146 will bediscussed with the understanding that the discussion of mounting ring146 also applies to mounting ring 148. An outer diameter of mountingring 146 is sized to closely fit into a bore 150 formed through hub 120.Bore 150 is closed by end plates 152, 154, each of which is secured tohub 120 by a plurality of bolts 156. A portion of mounting ring 146contacts an inner surface of end plate 152, but the inner surface of endplate 152 is spaced from an end of elastomeric bearing 142 and an end ofshaft 138. Thus, as the plurality of bolts 156 are tightened,elastomeric bearing 142 is selectively preloaded by the applied preloadforce against mounting ring 146. In some aspects, a face 143 ofelastomeric bearing 142 is tapered away from end plate 152 so thatelastomeric bearing 142 does not contact end plate 152. Mounting ring146 is keyed to hub 120 so that mounting ring 146 rotates with hub 120as the tail rotors flap. In order to remove, service or replace the tailrotor bearings of the present disclosure, the plurality of bolts 156 andend plate 152 are removed and then the trunnion 134, along withelastomeric bearings 142, 144 can slide out of bore 150.

The natural frequency of tail rotor system 110 can be tuned, in part, byaltering the design of elastomeric bearing 142, which acts as radialspring to allow some movement of tail rotors 112 relative to mast 111and to dampen vibrations within tail rotor system 110 (e.g., vibrationsand/or oscillations caused by operation of tail rotor system 110, tailrotor flapping, and the like). Altering parameters of elastomericbearing 142, such as the number of alternating layers, the type andthickness of rubber used, and the type and thickness of shim used,allows the effective spring rate of elastomeric bearing 142 to bemanipulated, which in turn helps tune the natural frequency of tailrotor system 110.

Determining the natural frequency of tail rotor system 110 involvescomplex mathematics that necessarily involves assumptions (e.g., atboundary conditions). As a result, it can be very difficult to preciselydesign a tail rotor system that avoids all dynamic loading issues, suchas resonance. In order to check for dynamic loading issues, workingprototypes of tail rotor systems are built and tested. After testing, itmay become apparent that dynamic loads exist that can cause prematurewear. By way of example, it was determined that, in some instances,elastomeric bearings 142, 144 of hub 120 were sometimes wearingprematurely. After careful consideration, the inventors determined thatpremature wear of elastomeric bearings 142, 144 was due to axialoscillations of elastomeric bearings 142, 144 (e.g., oscillations alongthe bore of elastomeric bearings 142, 144). As noted above, elastomericbearings 142, 144 act as radial springs. While the configuration ofelastomeric bearings 142, 144 provides resistance in the radialdirection (i.e., toward a central axis 139 through shafts 138, 140),elastomeric bearings 142, 144 provide comparatively little resistance inthe axial direction (i.e., parallel to central axis 139 through shafts138, 140). This lack of resistance in the axial direction is by designas normal operational loads do not present loading in the axialdirection. However, in some instances dynamic loading can create anoscillatory load in the axial direction. This oscillatory load canprematurely wear elastomeric bearings 142, 144 because elastomericbearings 142, 144 are not designed to withstand axial oscillatory loads.

Referring now to FIG. 5, an improved hub 120′ is illustrated incross-section. Improved hub 120′ includes many of the same parts as hub120. It should also be understood that hub 121 would similarly includethe improvements of improved hub 120′. Parts that are unchanged fromFIG. 4 are numbered the same in FIG. 5. Improved hub 120′ includes endbearings 158, 160 that abut ends of shafts 138, 140, respectively. Insome aspects, end bearings 158, 160 are sized so that no or minimalcontact is made with faces 143, 145 of elastomeric bearings 142, 144.Minimal contact is defined herein to mean that less than 5% of thesurface area of the faces of end bearings 158, 160 contact faces 143,145 of elastomeric bearings 142, 144, respectively. No or minimalcontact between end bearings 158, 160 and elastomeric bearings 142, 144is desirable because such contact could alter the performance ofelastomeric bearings 142, 144 in the radial direction.

As discussed above relative to FIG. 4, dynamic loading can create anoscillatory load in the axial direction of elastomeric bearings 142,144. For example, operation of tail rotor system 110 sometimes resultsin resonance at fundamental frequency of 1/rev. This resonance cancreate an axial oscillatory load that prematurely wears elastomericbearings 142, 144 because elastomeric bearings 142, 144 are not designedto withstand axial oscillatory loads. To reduce wear of elastomericbearings 142, 144, end bearings 158, 160 are used to adjust the springrate of improved hub 120′ along central axis 139 to prevent resonance atthe fundamental frequency. Changing the spring rate along central axis139 changes the natural frequency of improved hub 120′ along centralaxis 139 to avoid axial oscillatory loads, reducing wear of elastomericbearings 142, 144 caused by the axial oscillatory loads. Thus, byemploying end bearings 158, 160, the service life of elastomericbearings 142, 144 is increased.

In some aspects, end bearings 158, 160 are elastomeric bearings thatinclude alternating layers of rubber and metallic shims. In suchaspects, each layer of end bearings 158, 160 is oriented normal tocentral axis 139. This orientation of the layers provides resistance inthe axial direction (i.e., parallel to central axis 139) while providingminimal interference with the radial resistance provided by elastomericbearings 142, 144. Minimizing the radial resistance of end bearings 158,160 permits the design of elastomeric bearings 142, 144 to remainunchanged so as to not require other changes to the design of improvedhub 120′.

Compared to the design of hub 120, improved hub 120′ requires additionalspace to accommodate end bearings 158, 160. This additional space isprovided through the use of domed plates 166, 168. Domed plates 166, 168include domed portions that extend away from trunnion 134 to provideadditional space within improved hub 120′ for end bearings 158, 160.

As illustrated in FIG. 5, end bearings 158, 160 include retainers 162,164, respectively, that extend through domed plates 166, 168,respectively. Retainers 162, 164 removably secure end bearings 158, 160to domed plates 166, 168. Retainers 162, 164 ensure that end bearings158, 160 are properly located within improved hub 120′ duringinstallation and prevent end bearings 158, 160 from falling freely outof improved hub 120′ when domed plates 166, 168 are removed forservicing improved hub 120′.

The term “substantially” is defined as largely but not necessarilywholly what is specified (and includes what is specified; e.g.,substantially 90 degrees includes 90 degrees and substantially parallelincludes parallel), as understood by a person of ordinary skill in theart. In any disclosed embodiment or aspect, the terms “substantially,”“approximately,” “generally,” “around,” and “about” may be substitutedwith “within [a percentage] of” what is specified, where the percentageincludes 0.1, 1, 5, and 10 percent.

The foregoing outlines features of several aspects so that those skilledin the art may better understand the aspects of the disclosure. Thoseskilled in the art should appreciate that they may readily use thedisclosure as a basis for designing or modifying other processes andstructures for carrying out the same purposes and/or achieving the sameadvantages of the aspects introduced herein. Those skilled in the artshould also realize that such equivalent constructions do not departfrom the spirit and scope of the disclosure, and that they may makevarious changes, substitutions and alterations herein without departingfrom the spirit and scope of the disclosure. The scope of the inventionshould be determined only by the language of the claims that follow. Theterm “comprising” within the claims is intended to mean “including atleast” such that the recited listing of elements in a claim are an opengroup. The terms “a,” “an” and other singular terms are intended toinclude the plural forms thereof unless specifically excluded.

What is claimed is:
 1. An apparatus comprising: a body configured torotate about an axis; a trunnion disposed within the body; a shaftextending radially from the trunnion; and an end bearing disposedradially relative to an end of the shaft opposite the trunnion.
 2. Theapparatus of claim 1, wherein the end bearing is an elastomeric bearingcomprising alternating layers of rubber and metal shims.
 3. Theapparatus of claim 2, wherein the alternating layers of each of the endbearing are oriented normal to an axis passing through a central axis ofthe shaft.
 4. The apparatus of claim 1, comprising: an end plate; andwherein the end plate comprises a dome; and wherein at least a portionof the end bearing is disposed within the dome.
 5. The apparatus ofclaim 4, wherein the end bearing comprises a retainer that connects theend bearing to the end plate.
 6. The apparatus of claim 1, furthercomprising: an elastomeric bearing, the elastomeric bearing beingdisposed on the shaft; and a mounting ring, the mounting ring disposedbetween the elastomeric bearing and an inner wall of the body.
 7. Theapparatus of claim 6, wherein: the bearing comprises a face thatcontacts an end of the shaft; and the face of the end bearing does notcontact the elastomeric bearing.
 8. The apparatus of claim 6, whereinthe shaft is frustoconical in shape and the elastomeric bearingcomprises alternating layers of shims that form concentric rings withthe frustoconical shaft.
 9. The apparatus of claim 1, wherein the endbearing comprises a retainer that connects the end bearing to an endplate.
 10. An apparatus comprising: a trunnion disposed within a bodyconfigured to rotate about an axis; an elastomeric bearing disposed on ashaft extending radially from the trunnion; a mounting ring disposedbetween the elastomeric bearing and an inner wall of the body; an endplate secured to the body and comprising a dome; and an end bearingdisposed between the shaft and the end plate.
 11. The apparatus of claim10, wherein the end bearing is an elastomeric bearing comprisingalternating layers of rubber and metal shims.
 12. The apparatus of claim11, wherein the alternating layers of each of the end bearing areoriented normal to an axis passing through a central axis of the shaft.13. The apparatus of claim 10, wherein at least a portion of the endbearing is at least partially disposed within the dome of the end plate.14. The apparatus of claim 10, wherein end bearing comprises a retainerthat connects the end bearing to the end plate.
 15. The apparatus ofclaim 10, wherein: the end bearing comprises a face that contacts an endof the shaft; and the face of the end bearing does not contact theelastomeric bearing.
 16. The apparatus of claim 10, wherein the shaft isfrustoconical in shape and the elastomeric bearing comprises alternatinglayers of shims that form concentric rings with the frustoconical shaft.17. A method comprising: providing a body configured to rotate about anaxis; placing a trunnion within the body, the trunnion comprising ashaft; placing an elastomeric bearing on the shaft; placing an endbearing adjacent to the shaft; and securing an end plate to the bodysuch that the end bearing is disposed between the end plate and theshaft; and wherein the end bearing adjusts a spring rate along a centralaxis passing through a center of the elastomeric bearing to move anatural frequency along the central axis away from a fundamental naturalfrequency.
 18. The method of claim 17, wherein the end bearing comprisesa retainer that connects the end bearing to the end plate.
 19. Themethod of claim 17, wherein: the end bearing comprises a face thatcontacts the shaft; and the face does not contact the elastomericbearing.
 20. The method of claim 17, wherein the end plate comprises adome that houses at least a portion of the end bearing.